Various publications, including patents, published applications, technical articles and scholarly articles are cited throughout the specification. Each of these cited publications is incorporated by reference, in its entirety and for all purposes, in this document.
In the simplest terms, the spine is a column made of vertebrae and discs. The vertebrae provide the support and structure of the spine while the spinal discs, located between the vertebrae, act as cushions or “shock absorbers.” These discs also contribute to the flexibility and motion of the spinal column. Over time, the discs may become diseased or infected, may develop deformities such as tears or cracks, or may simply lose structural integrity (e.g., the discs may bulge or flatten). Impaired discs can affect the anatomical functions of the vertebrae, due to the resultant lack of proper biomechanical support, and are often associated with chronic back pain.
Several surgical techniques have been developed to address spinal defects, such as disc degeneration and deformity. Spinal fusion has become a recognized surgical procedure for mitigating back pain by restoring biomechanical and anatomical integrity to the spine. Spinal fusion techniques involve the removal, or partial removal, of at least one intervertebral disc and preparation of the disc space for receiving an implant by shaping the exposed vertebral endplates. An implant is then inserted between the opposing endplates.
Spinal fusion procedures can be achieved using a posterior or an anterior approach, for example. Anterior interbody fusion procedures generally have the advantages of reduced operative times and reduced blood loss. Further, anterior procedures do not interfere with the posterior anatomic structure of the lumbar spine. Anterior procedures also minimize scarring within the spinal canal while still achieving improved fusion rates, which is advantageous from a structural and biomechanical perspective. These generally preferred anterior procedures are particularly advantageous in providing improved access to the disc space, and thus correspondingly better endplate preparation.
Some of the common problems with spinal implants include movement or expulsion of the implant once inserted between adjacent vertebrae. In particular, when the flexible tissue (the annulus) connecting the disks is severed in the surgical procedure additional vertical and lateral instability in the joint is induced. In order to reduce implant movement or expulsion from between the vertebral bodies, spinal implants may be affixed to adjacent vertebrae, for example, using additional fixation elements, such as screws. The use of additional fixation outside of the joint space, for example, by using screws and plates, screws and rods, or screws alone can limit the amount of displacement that occurs as the vertebra move away from one another reducing movement and activity. Unfortunately, screws can loosen, back out, and even break over time.
A number of screw retention or secondary screw fixation devices are available to try to combat the problem of back out. For example, a screw locking plate and fastener assembly may be placed over the heads of the screws or a snap may be embedded into the implant body. Typical screw retention devices rigidly fix the screws within the device. This rigidity does not allow for movement of the screws, however, and can result in increased loading in the joint space. In other words, the loading can create pressure points where the screws are located and can produce undesired bone remodeling at those locations. Similarly, implants having aggressive teeth or ridges can remodel the bone around these sharp features providing instability and movement in the joint assembly. Rigid fixation, increased loading and pressure points, and movement and instability of the implant can result in mechanical failure of the screws. Mechanical failure of the screws and associated pieces of the screw retention devices (e.g., screw locking plate, etc.) places the patient at risk for unsecured screws in the vertebral disk space.
One attempt at addressing the screw back-out problem is provided in U.S. Pat. No. 6,241,731, which describes a plate and screw assembly for fixing bones wherein the screw includes a retainer fitted about the head of the screw for limiting axial movement of the screw after installation with the plate. The plate has at least one orifice which defines a cavity for receiving the screw head and retainer therein such that there should be free pivotal movement, but limited axial movement, of the screw to prevent the screw from moving out of the orifice of the plate. Rather than having a complete annular ring shape, the retainer has a “C” shape with a uniformly-sized, axially-oriented slot which enables the retainer to slightly yield to compression forces or slightly flex in response to expansion forces, which facilitates insertion and retention of the coupled screw head and retainer in the orifice of the plate.
Construction and use of the plate and screw assembly of U.S. Pat. No. 6,241,731 has been found to be impracticable. For example, it was found that the retainer could not be made with an effectively-sized slot that could at the same time allow the retainer to be placed around the screw in a manner that prevented axial movement of the screw and support load forces. A proper balance between strength and flexibility could not be achieved with this plate and screw design. When the retainer was constructed of material strong enough to secure the implant and support the expected load forces, the width of the slot required to allow coupling with the screw head without damage to the retainer body was too wide to securely and pivotably retain the screw head therein. Conversely, when the width of the slot was made narrow enough to ensure that the screw head was pivotably secured in the retainer, the retainer body cracked or broke when the retainer and screw head were brought together, i.e., the retainer was not strong and could not support assembly forces, let alone load forces.
Thus, a need remains for a screw fixation device that can secure a screw in an implant device without the risk of back out or fracture of the screw, but does not create any of the problems mentioned above for traditional screw retention devices. It is desirable to have a screw fixation device that at the same time minimizes or prevents axial movement of the screw, and allows full pivotal movement of the screw relative to the implant device after surgical implantation, and furthermore, the fixation device must be capable of being coupled to the screw without structural damage or failure to the fixation device.